Fixes #183: Allow reading output pins (#986)

* Allow reading output pins

* Add test to verify ability to read from output pins

* Extend test to verify pin value can be read after writing

src/System.Device.Gpio.Tests/GpioControllerTestBase.cs

@@ -116,12 +116,17 @@ namespace System.Device.Gpio.Tests
         }
 
         [Fact]
-        public void ThrowsIfReadingFromOutputPin()
+        public void CanReadFromOutputPin()
         {
             using (GpioController controller = new GpioController(GetTestNumberingScheme(), GetTestDriver()))
             {
                 controller.OpenPin(OutputPin, PinMode.Output);
-                Assert.Throws<InvalidOperationException>(() => controller.Read(OutputPin));
+
+                controller.Write(OutputPin, PinValue.Low);
+                Assert.Equal(PinValue.Low, controller.Read(OutputPin));
+
+                controller.Write(OutputPin, PinValue.High);
+                Assert.Equal(PinValue.High, controller.Read(OutputPin));
             }
         }
 

src/System.Device.Gpio/System/Device/Gpio/GpioController.cs

@@ -172,11 +172,6 @@ namespace System.Device.Gpio
                 throw new InvalidOperationException("Can not read from a pin that is not open.");
             }
 
-            if (_driver.GetPinMode(logicalPinNumber) == PinMode.Output)
-            {
-                throw new InvalidOperationException("Can not read from a pin that is set to Output mode.");
-            }
-
             return _driver.Read(logicalPinNumber);
         }